when I go into the cpanel administrative interface, and force it to synch the zones, it does so immediatly... but I need to know how frequently it synchs them automatically so that our clients know how long they must wait before their dns changes are replicated through our entire dns infrastructure. Do I need to rig this up manually with a cron job?

I believe that you are referring to the TTL of the individual zone file. That information is used for replicating zone files when using the master-slave relationship, but the way that I understand it, that stuff isn't relavant to this custom DNS clustering.